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INTRODUCTOilY   ESSA\.                                    HI
of Afghanistan, and the winter rains of the lower hills and of
the plains at the foot of the mountains. These last arc irre-
gular in amount and period, arid dependent perhaps on local
disturbances of the great current, the causes of which are
still obscure and require careful investigation, During the
south-west monsoon, a similar return current from Siberia
and Tartary probably flows almost uniformly from the north-
ward, at a very great elevation, and joins the ascending cur-
rent from the plains of India.
When the causes and direction of the periodical winds arc
clearly indicated, there is no difiiculty in understanding why
it is that in some parts of ludia the climate is always moist,
both monsoons being rainy, while in others one monsoon only
is rainy, and in others again there is 110 rain at any pcried of
the year. The only permanently rainy province is the Ma-
layan peninsula, and the only absolutely arid ones arc Sind
and the neighbouring deserts of the Panjab. Throughout the
greater part of India one monsoon is rainy, and that gene-*
rally the south-west one, blowing from May or June till the
end of September.
The amount of rain varies prodigiously in different parts of
India, from almost none to six hundred inches, >>ut the details
must be reserved for notice under the several districts. It
is very essential to bear in mind that the rain-fall affords no
direct criterion of the humidity of any climate, for the atmo-
sphere may be saturated with moisture without any preci-
pitation taking place. The influence upon vegetation of the
vapour suspended in the air, and thus brought in contact with
every surface of the foliage, is most important, and can only
be ascertained by means of daily observations with the hy-s
grometcr. This instrument is indeed, generally speaking, of
far more importance to the botanist than the thermometer;
the distribution of tropical plants especially, in so far as it is
influenced by climate, being so by its moisture*
* To make our moaning clearer, "we may say that any part of the tropics is
hot enough for the growth of a tropical plant, hut that whole natural orders,